The present invention relates to a small electric motor comprising a stator, a rotor with a rotor shaft and a projecting shaft end of the rotor shaft which has arranged thereon an element to be driven. The invention also relates to a method for dynamically balancing such an electric motor.
In particular with applications where relatively large components are to be rotated by small electric motors, vibrations might arise that are disadvantageous for specific uses. Such problems will in particular arise when the components are directly arranged on the rotor shaft of the motor and are made relatively flat, but have a large diameter often exceeding the diameter of the motor. Such components may e.g. be fan impellers, filter discs or any type of component to be driven by an electric motor. As for such components that are large in comparison with the constructional size of the electric motor and are only seated on a small portion of the rotor shaft, axial runouts play a very great role in the generation of undesired vibrations. Another drawback inherent to most applications is that the rotational speeds produced by small electric motors are often very high, whereby the vibrations are further increased. Flat type motors are used for some applications for reasons of space, as a consequence of which the rotor cannot be balanced because of its small length.
It is now the object of the present invention to provide a small electric motor of the above-mentioned type with a low-vibration running.
According to the invention this object is achieved in that the other shaft end of the rotor shaft is also designed as a projecting shaft end and a balancer is arranged on said other shaft end with an arrangement and mass or weight distribution predetermined for dynamically balancing the element to be driven. In all of the known cases in which drivable elements of a relatively large size and most of the time of a considerable mass or weight in comparison with the motor are to be moved by small electric motors, a shaft end for the arrangement of said element only projects at one side. It now seems to be a very unusual measure that at the other side of the electric motor that has so far had no function assigned to it in such types of motors and has normally been closed by an end shield, a projecting shaft end is now arranged for mounting a balancer thereon. Often, the confined installation space only plays a role at the side at which the element to be driven, for instance a fan impeller, a filter plate, or the like, is arranged. At the other side the confined installation space is often of secondary importance so that the balancer can also be given a very large size and equipped with an adequate mass or weight. This permits an excellent dynamic balancing operation. Apart from a low-noise or low-vibration running, the load on the bearings is also reduced, which in turn will increase the service lives thereof. The balancer can be designed such that it is suited for the mounting of additional weights or masses and also for the removal of parts thereof. With such a measure it is inter alia possible to further increase the considerable mass or weight of the elements to be driven. Moreover, an axial runout of the rotor or of the element which is to be driven and arranged on the rotor and which leads to a dynamic unbalance (gyroscopic moment) can very well be compensated for by balancing in a second plane at a certain axial distance. The axial runout had to be kept very small in formerly used elements to be driven. This entailed high demands on the manufacture of said members and on the mounting device. High manufacturing costs were the result thereof. Thanks to the use of the balancer the demand made as to axial runout tolerance need not be so high, whereby the manufacturing and mounting costs of the element to be driven can also be reduced. The projecting shaft end is here in particular meant to be a shaft end extended beyond the bearing point.
One variant of the invention has a particularly advantageous effect in that the element to be driven is a disc-like body whose diameter is larger than the diameter of the rotor or the stator. In such disc-like bodies, the surface getting into contact with the rotor shaft is relatively small so that axial runouts might also be produced during mounting operations, e.g. gluing, pressing or similar installation possibilities, on the rotor shaft. A dynamic balancing operation by means of a balancer of a suitable size is excellently suited for such thin large-diameter elements to be driven.
The above advantages are in particular enhanced in variants in which the stator and the rotor are made flat. Such constructions, which are designated as flat type motors (pancake motors, cup motors), have so far not offered any possibility of using balancers, no matter of what type. The otherwise closed back side of such motors is now penetrated according to the invention by a projecting shaft end for mounting the balancer.
To observe axial installation lengths that are as small as possible at the side of the balancer, the balancer may preferably have the shape of a disc. In disc-like balancers a satisfactory result can readily be achieved by slightly changing the arrangement and mass or weight distribution.
Advantageously, the balancer may be made from a material having a high specific weight, such as brass, steel or copper, and the targeted mass or weight distribution may be effected by means of recesses, produced in particular by machining. This means, for instance, that a simple sleeve can be anchored to the rotor shaft and is then provided at specific points with recesses or with a single recess in accordance with the desired balancing operation. The location for the recess can rapidly be detected by means of suitable machines which could then form or incorporate the recess at the same time.
However, it is also possible to design the balancer as a carrier element in or on which the balancing weights can be arranged and/or mounted. The balancing weights can be mounted variably on the carrier element with respect to their quantity and also their place. The balancing weights can be graduated to a very fine degree, so that corresponding results can be achieved with a satisfactory accuracy.
For instance at least one paste with a suitable specific weight can be used as the balancing weight. For instance, if the carrier element is designed as a hollow body with various receiving chambers, targeted balancing results can be achieved by introducing predetermined pastes with a respectively suited specific weight. The composition of the paste will then follow from the necessary mass or weight. Additives, such as metal powder, may be contained.
However, it is also possible that the balancing weight comprises metal balls. These may also be arranged in a paste. Metal balls can be produced with different diameters and have a specific weight in accordance with material and size. Furthermore, they can be pre-positioned on the carrier element and displaced by suitable means for adjusting purposes so that sometimes the mass or weight has just to be displaced and need not be added or removed.
In a particularly advantageous development a housing may be provided, the shaft ends may respectively project from the housing and the element to be driven or the balancer may be arranged outside the housing. As for the construction of such a small electric motor, attention has just to be paid that a shaft stub on which the balancer can be arranged is projecting at the other end. Additional measures are not needed, so that the otherwise compact constructional shape of the motors can be maintained.
Although the most different mounting bodies can be used as elements to be driven, said body is formed by a fan impeller according to one embodiment. The use of fan impellers is wide-spread and their applications are numerous. It is now possible to operate relatively large fan impellers by relatively small motors without any objectionable effects caused by vibrations.
However, it is also possible to use an optical filter disc as the element to be driven. Such filter discs often consist of different materials arranged as a composite system, so that an unbalance or axial runout must always be expected because of the type of production. Such an unbalance or axial runout can be sufficiently compensated by the balancer, so that even high speeds can be employed.
Furthermore, the present invention relates to a method for the dynamic balancing of small electric motors. The method is meant to allow for a simple balancing of rotor shafts with relatively large elements to be driven without any need for a disadvantageous interference with the construction of the small electric motor. To this end a balancing element is arranged on another projecting shaft end of the rotor shaft that is axially spaced from the shaft end comprising the element to be driven, and a dynamic balancing of the entire rotor is performed by removing or mounting or displacing at least one balancing weight on the balancing element. This means that balancing can respectively be carried out beyond or outside the bearing point or points on the projecting end portions of the rotor shaft.
Preferably, the dynamic balancing operation is carried out with the rotor being arranged inside the stator. The small electric motor is thus completely mounted, including the element to be driven, so that there are numerous possibilities of mounting and designing balancers (also several balancers at the same time) by a sliding movement onto the opposite projecting shaft end. The dynamic balancing operation is thereby simplified enormously. With the aid of modern balancing machines one can very rapidly determine where and with which weight distribution the balancer is to be arranged on the shaft end.
Advantageously, the at least one balancing weight can be removed by removing material on the balancing element. In such a case it would be possible to press or glue a metal sleeve, e.g. of steel, onto the projecting shaft end and following a subsequent unbalance determining operation a recess is incorporated at a specific place by drilling, milling or other machining or eroding operations, etc.